On October 23rd, the University of Maine’s floating wind turbine was the star attraction at the American Wind Energy Association (AWEA) Off-Shore Wind conference, which took place in Providence, Rhode Island.
A University of Maine (UM) team launched the first American floating wind turbine, VolturnUS, on May 31st off the coast of Maine that opens up possibilities for wind farm development on the U.S. Atlantic and Pacific coasts, as well as in Hawaii.
The prototype is a 65-foot-tall turbine prototype, which is a one-eighth-scale version of a 6-megawatt full-scale turbine that UM hopes to deploy off Maine’s coastline in 2016. The floating platform operates at depths of hundreds of feet similar to offshore oil platforms.
“There are many firsts here today,” said Habib Dagher, director of the University of Maine’s Advanced Structures and Composites Center. “Not only is VolturnUS the first floating wind turbine in the U.S., it’s the first offshore wind turbine tower to be completely made of composites, and the first concrete/composite floating turbine hull in the world.”
At the AWEA conference, Dagher told AJOT:” The concrete production process is less expensive to fabricate in New England than importing fabricated steel hulls from Asia, the low-cost producer of large offshore fabricated steel structures. We have already observed that producing components from concrete can be less expensive than steel in the construction of bridges located in highly corrosive environments, and our intention is to migrate that technology into offshore wind turbine structures.”
Dagher said that a major innovation in the VolturnUS is reducing the weight of the wind turbine tower, which makes the entire floating hull lighter and more stable. This means wind turbine mechanisms are less subject to wear and tear in ocean conditions: “For a 6 MW turbine, the steel tower can weigh as much as 400 to 600 tons, but weighs far less with composites. This means the floating turbine is less top heavy and more stable.”
Paul Williamson, director of the Maine Wind Industry Initiative based in Portland, Maine says the cost of power proposed to be generated by small pilot offshore floating wind demonstration projects will be expensive and cost over 20 cents per kilowatt hour compared to the current 7 cents per kilowatt hour for conventional power. However, as projects are scaled up in size and move towards commercialization, developers expect to produce power at 10 cents per KWH.
Williamson says, Maine is developing a comprehensive port logistics strategy to support what it believes will be a major build out of offshore wind assets with a starting target of 1,000 MW and the creation of 2,000 jobs. The state has official goals of eventually building 5000 MW of offshore wind.
Williamson told the AJOT that for a full commercial scale build out the port logistics investment will require $300M to $400M. He says that several regions in Maine are being looked at for port development:
• Searsport/ Upper Penobscot Bay
• Kennebec River region
The University of Maine’s VolturnUS unit was built in components at the University’s Advanced Structures and Composites Laboratory, then assembled by Cianbro, an engineering construction company that partnered with the University to fabricate the floating wind turbine. Dagher noted the following improvements:
Semi Submersible Hull
The UM design was found to be the most advantageous for the Gulf of Maine conditions compared to other designs such as the turbine-mounted-on-a- spar buoy promoted by the Norwegian oil and gas giant, Statoil.
Concrete Replaces Steel Foundation
The semi-submersible base is made of concrete strengthened with composite materials, which eliminates the need for steel construction. Dagher said: “In Europe, the world leader in offshore wind, projects have a typical life of about twenty to twenty-five years. The VolturnUS hull is designed to be re-used for three to four such life cycles. That’s why we developed the concrete hull.” The concrete hull eliminates steel welding requirements and related stress-corrosion-fatigue problems, which limit the fatigue life of steel structures. The composite materials tower design borrows from the aircraft industry, but is now being tested in a marine setting. The durability of these designs could result in significant lifetime cost saving opportunities compared to steel-based floating platforms. A European designer has noted that man hours for wind turbine fabrication in Europe are skyrocketing because of insurance and marine certification requirements for super quality controls to insure structural integrity. The UM Advanced Structures and Composite Center is providing a new paradigm for offshore wind construction that could help the United States develop a cost-effective offshore wind industry.
The composite tower reduces topside weight, and eliminates steel corrosion and fatigue. It creates the potential for a more stable and cost-effective structure.
Floating turbines may replace fixed foundation structures. Starting in 1991, construction in Europe has largely focused on shallow water where the turbine base is fixed to the seabed. These wind turbines require in-water construction of foundations and costly installation methods using jack up vessels, which cost over $100M/day to operate. While such designs with turbines fixed to the seabed have been proposed for several U.S. Atlantic coast projects, the UM project may strengthen the case for building floating turbines moored to the sea bed in deep water, such as off the Maine coast. Deepwater semi-submersible structures are commonly used in the oil and gas industry. Research shows that wind generation is increased when moving wind turbines further out to sea, where the wind is more powerful and steady. This opens up deep water wind farm opportunities on the U.S. Pacific coast and Hawaii.
Maintenance costs for the Volturn US should be less than steel structures. Dagher says the VolturnUS floats and has catenary mooring lines and anchors like ship moorings. It can be towed out to sea, connected to anchors and plugged into the grid. Minor repairs are done offshore. Under certain conditions, major repairs could be done more cost-effectively by towing the wind turbine back to port for work rather than performing expensive maintenance work at sea. Time will tell whether storm and wind will validate this claim.
One big unanswered question is how to integrate massive amounts of new offshore wind energy into the U.S. grid system. The wind farm development off the North German coast has run into problems integrating offshore wind into the power grid. The problem surfaced when a transmission operator baulked at paying for new transmission lines that connect offshore wind farms to German population centers. In the United States, the answer may lie with building long coastal offshore transmission lines under water. The U.S. Atlantic Wind Connection is proposing to build offshore transmission lines that will connect wind farms to the grid off the mid-Atlantic coast. It is expected to begin construction off the coast of New Jersey in 2016.